What Is the Specific Threat Known as a “Fork Attack” in the Context of SPV?

A fork attack against an SPV client involves a malicious miner creating a short, fraudulent side chain that includes a double-spend transaction. The attacker presents the block headers of this fraudulent chain to the SPV client, which, due to its reliance only on headers and Merkle proofs, might mistakenly accept the fraudulent chain as the valid one.

This allows the attacker to confirm a transaction to the SPV client while spending the same funds elsewhere on the true main chain.